In conventional methods of manufacturing a protein gel such as tofu (bean curd) in which an aqueous solution containing protein (hereinafter referred to as "protein-containing solution") is filled in a food container for curdling therein, a curdling chemical, transglutaminase or the like is added to the protein-containing solution, and is then heated.
FIG. 1 is a flowchart showing a first conventional method of manufacturing a protein gel, and FIG. 2 is a temperature profile in the first conventional method of manufacturing a protein gel. In FIG. 2, the abscissa represents time while the ordinate represents temperature.
As shown in FIGS. 1 and 2, in the first step, the temperature of a protein-containing solution is elevated from about 80.degree. C. to a temperature of 130-140.degree. C. at a time t1 to heat the solution for a few to 10 seconds for sterilization.
When the protein gel to be obtained is tofu, soymilk is used as the protein-containing solution. A process for manufacturing the soymilk is as follows. Soy beans are first washed and then immersed in an unillustrated immersion tank. Thereafter, the soy beans are crushed with an unillustrated grinder to obtain "go", a paste of crushed soy beans, which is then heated at a temperature of 98-105.degree. C. for 2-5 minutes. Thereafter, tofu refuse is removed from the "go" to obtain soymilk.
In the second step, a curdling chemical is added to the protein-containing solution at a time t2. In this step, gluconodeltalactone (GDL), magnesium chloride (MgCl.sub.2) or the like is used as the curdling chemical. The curdling chemical is added to the protein-containing solution after being passed through a bacteria-removing filter. The temperature of the protein-containing solution is maintained at 10.degree. C. to prevent the chemical reaction from proceeding, which would otherwise occur at higher temperatures.
In the subsequent third step, the protein-containing solution is mixed with the curdling chemical at a time t3 to obtain a mixture. In this step, the temperature of the protein-containing solution is also maintained at 10.degree. C. to prevent the chemical reaction from proceeding, which would otherwise occur at higher temperatures.
In the fourth step, the mixture is aseptically filled in an unillustrated food container at a time t4. In this step, the temperature of the protein-containing solution is maintained at 10.degree. C. to prevent the chemical reaction from proceeding, which would otherwise occur at higher temperatures.
In the fifth step, the mixture is heated at a temperature of 90-95.degree. C. for 30 minutes, at a time t5, for curdling the protein-containing solution in the mixture by the action of the curdling chemical.
A protein gel can be manufactured in this manner.
FIG. 3 is a flowchart showing a second conventional method of manufacturing a protein gel, and FIG. 4 is a temperature profile in the second conventional method of manufacturing a protein gel. In FIG. 4, the abscissa represents time while the ordinate represents temperature.
As shown in FIGS. 3 and 4, in the first step, a protein-containing solution is cooled from about 80.degree. C. to a temperature of 25-50.degree. C. at a time t11. To a cooled solution, transglutaminase is added. Transglutaminase is an enzyme serving as a catalyst in an acyl transferring reaction of a .UPSILON.-carboxyamide group, which is a glutamine residue in a peptide chain.
In the second step, the protein-containing solution is mixed with transglutaminase at a time t12. In this step, the temperature of the protein-containing solution is maintained at 25-50.degree. C. to prevent the catalytic reaction from proceeding by the presence of transglutaminase, which would otherwise occur at higher temperatures.
In the subsequent third step, the mixture of the protein-containing solution and transglutaminase is filled in an unillustrated food container at a time t13. In this step, the temperature of the mixture is also maintained at 10-60.degree. C., preferably at 25-50.degree. C., to prevent the catalytic reaction from proceeding by the presence of transglutaminase, which would otherwise occur at higher temperatures.
In the fourth step, the mixture is heated at a low-temperature of 40.degree. C., at a time t14, so as to accelerate the catalytic reaction by transglutaminase, thereby obtaining a protein gel.
In the fifth step, the protein gel is heated at a high-temperature of 90.degree. C., at a time t15, to inactivate transglutaminase.
A protein gel can be manufactured in this manner.
However, in the above-described conventional method of manufacturing a protein gel in which a curdling chemical is used, a mixture of a protein-containing solution and a curdling chemical must be heated at a temperature of 90-95.degree. C. for 30 minutes in the fifth step to curdle the protein-containing solution in the mixture. In another conventional method of manufacturing a protein gel in which transglutaminase is used, a mixture must be first heated at a low temperature of 40.degree. C. in the fourth step to accelerate the catalytic reaction by transglutaminase, and then heated at a high temperature of 90.degree. C. in the fifth step to inactivate the transglutaminase. These conventional methods, therefore, have the drawback that considerable energy is consumed in the manufacturing process, resulting in high manufacturing costs.
An object of the present invention is to solve the problems of the above-described conventional methods of manufacturing a protein gel and to provide a method of manufacturing a protein gel which can reduce the manufacturing costs.